Apart from when breakdowns occur, millers nowadays produce finished products whose purity, as regards foreign substances content, is almost absolute. The product passes through a large number of screening stages already when the ground material is being produced. Unexpectedly, however, check screening operations at the customers can disclose a troublesome foreign content or awkward foreign constituents. Therefore, this requires a further final screening before the further processing of materials of the flour or semolina type. This kind of entry-stage final screen at the premises of a consumer of foodstuffs or animal feed, or of food mill products, has to meet very specific requirements. Basically, out of relatively large delivery quantities, for example 8,000 kg of flour per hour, perhaps only a few grams of impurities have to be screened out. The final screening is to be particularly reliable before further processing is begun. It is often necessary to choose a situation for the screening machine on the way along the conveying line. The final screen can be arranged within a for example already existing pneumatic conveying line, but then there is a requirement to construct the screen suitably to obviate having considerable resistance, which would increase the power required for pneumatic conveyance. A characteristic category has developed here of the so-called "screening machine in the pneumatic conveyance line", emanating from the proposal in the old German patent specification No. 867 193. This old publication proposed arranging a beater device with a high rotational speed within a cylindrical screen shell, and effecting the further conveyance of the flour freed from husks through an annular cylindrical duct with the air flow. However, the emphasis of that apparatus is on functioning as a bran centrifuge, which is a typical apparatus in process technology in milling. In DE-OS No. 2 121 726 the said apparatus was further developed as regards the actual function of final screening of large quantities of flour which for example have to be transferred from a transport vehicle into silos. The proposal is put forward to situate the screen apparatus at the end of a pneumatic conveying conduit. Instead of the traditional beater device in bran centrifuges a rotor provided with longitudinal blades is selected, and the material for screening is conducted through a disc against the screen shell.
In a further development of the final screen apparatus according to DE-OS No. 2 338 909 it is proposed to employ unilateral mounting for the rotor, which throws the material outwards. The feeding pneumatic conveying conduit can be extended in the axial direction from the machine side opposite from the drive to beyond the middle of the screen cylinder interior space. Disadvantages which result here are unfavourable flow conditions for the material-air mixture, and a concept which as far as machine construction is concerned is suitable only for relatively small apparatus, owing to the unilateral mounting. Access to the screen is made difficult.